Engine oil is used for lubricating and cooling crank mechanism and various engine parts. Accordingly, viscosity property, an extreme pressure property of lubrication performance under high loads, oxidation stability, a friction property and the like may be required for the engine oil. Typically, base oil and additives having suitable properties may be mixed in the engine oil composition to improve such performances.
Recently, regulations on vehicle exhaust gas such as carbon dioxide have become stricter for efficient energy use and global warming prevention. In response to these environment regulations, engine oil for improving fuel efficiency and reducing the energy loss of engines has been actively developed. Particularly, efforts for improving efficiency by lowering a friction coefficient with the use of low friction additives have been continuously made to reduce the friction in the mixed lubrication areas of engine valves or piston parts.
For example, in the related arts, a diesel engine oil composition has been provided and the composition includes zinc dithiophosphate (ZnDTP) and glycerol monooleate (GMO) in base oil as an antiwear agent in addition to a viscosity index improver and a detergent dispersant. However, zinc dithiophosphate may be readily decomposed at elevated temperature condition thereby increasing in the amount of ash components produced and further reducing endurance due to valve deposits in the engine.
In addition, an engine oil composition has been developed and the engine oil composition includes highly-concentrated molybdenum dithiocarbamate (MoDTP) having molybdenum content of about 9 to 15% by weight, zinc-dialkyldithiophosphate, and a calcium-based or a calcium borate-based salicylate. However, when a salicylate metal salt detergent dispersant and molybdenum dithiocarbamate are used together, metal salt deposits may be generated due to the combustion at elevated temperature condition such that its application as engine oil may be limited.
Furthermore, in another example, a gasoline engine oil composition including a molybdenum dithiocarbamate (MoDTP) or glycerol monooleate (GMO) friction modifier together with a zinc dithiophosphate (ZnDTP) antiwear agent has been introduced. In other related arts, a technology of using MoDTP or GMO as an auxiliary additive has been developed to reduce the friction which is not reduced when ZnDTP is used alone. However, such technology does not provide combined use of MoDTP and GMO for simultaneously improving frictional resistance, wear resistance and fuel efficiency.
Particularly for large size diesel engines with large loads and low speed, lowering a friction coefficient may be limited by adding a low friction agent due to relatively large mixing and fluid lubricating area parts. In addition, when viscosity is lowered for additional fuel efficiency improvement, wear resistance may deteriorate as thickness of an oil film decreases when the viscosity decrease.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.